1. Field of the Invention
The field of art to which the claimed invention pertains is liquid-gas absorptive separations. More specifically, the claimed invention relates to a process for the absorptive removal of sulfur dioxide from a gaseous mixture by means of scrubbing liquor.
2. Description of the Prior Art
Due to the ever increasing concern about air pollution, great efforts have been expended in recent years toward the development of processes to reduce the pollutants introduced into the atmosphere from various industrial operations. One of the most onerous of these pollutants is sulfur dioxide which is present in the stacks or flue gases from various operations. For example, SO.sub.2 is a primary pollutant released into the atmosphere in the production of sulfuric acid. Also, great quantities of SO.sub.2 are produced and passed to the atmosphere in the generating of power, particularly where the fuel used is high in sulfur content.
It has only been known to the art to abate sulfur dioxide pollution of the atmosphere. The so-called "wet process" is probably the most commonly used process to accomplish such abatement. In the wet process the SO.sub.2 containing gas mixture is contacted or "scrubbed" with water in which is dissolved or slurried a reagent that chemically reacts with the SO.sub.2, thus effecting the removal of the SO.sub.2 from the gas mixture and incorporating the sulfur originally contained in the SO.sub.2 onto a non-volatile compound dissolved in the scrubbing water. The sulfur is ultimately removed from the system by the discarding of the scrubbing water in which it is contained, or by removal from the water by further processing.
One reagent known for use in the scrubbing solution for the wet scrubbing process is claimed in U.S. Pat. No. 4,134,961 and comprises a mixture of sodium aluminate and sodium hydroxide with the mole ratio of sodium to aluminum of from about 1:1 to about 2:1. In the scrubbing process utilizing that reagent the SO.sub.2 reacts with both the sodium hydroxide and sodium aluminate to form both sodium and aluminum sulfur containing compounds.
The reagent perhaps best known to the art for use in the scrubbing solution for the above wet process is alkali metal carbonate. The alkali metal carbonate reacts with the SO.sub.2 to yield alkali metal sulfite and CO.sub.2. Use of the alkali metal carbonate is particularly advantageous because solutions of that compound are commonly available as process-waste streams from industries such as metal production, textiles and paper making. The use of such streams not only cuts operating costs but also permits economies in the capital costs of wet processes because it eliminates the need for equipment to store and mix reagents obtained from other sources. Examples of prior art wet SO.sub.2 scrubbing processes which utilize alkali metal carbonates are taught or claimed in U.S. Pat. Nos. 3,962,410; 3,963,825; 3,972,980; 3,984,529; and 3,987,147.
I have observed that certain inexpensive sources of soda liquor (sodium carbonate solutions) for use in wet SO.sub.2 scrubbing processes contain quantities of silica which, when the pH of the liquor is lowered due to SO.sub.2 absorption, precipitate and form SiO.sub.2 deposits on the internals of the scrubbing apparatus. These deposits, which are essentially solid rock, interfere with the operation of the apparatus by eventually plugging the pipes and liquid circulating means which are a part of the apparatus. There is no practical way of removing these deposits.
I have found a means of minimizing the deleterious effects of the presence of silica in the scrubbing liquor of the wet SO.sub.2 scrubbing process.